Conventionally, a general color solid-state image sensing device detects photo signals of specific wavelength ranges obtained via color filters, which are arranged on respective pixels. Hence, only a part of incident light reaches a semiconductor where photoelectric conversion is carried out, and a signal output decreases compared to a signal converted without color filters. A proposal that solves such a signal loss problem has been made by U.S. Pat. No. 4,613,895 “Color Responsive Imaging Device Employing Wavelength Dependent Semiconductor Optical Absorption”.
According to this proposal, threefold photoelectric conversion units are layed in a semiconductor in depth, and three kinds of signal charges accumulated in these photoelectric conversion units are independently transferred and read out using triple CCDs which are also formed in the semiconductor in depth.
FIG. 13 shows a representative embodiment of the above proposal. Referring to FIG. 13, reference numeral 51 denotes a semiconductor substrate; 52, an insulating film formed on the interface of the semiconductor substrate 51; and 53, an electrode formed on the insulating film 52. The semiconductor substrate 51 is formed by stacking p-, n-, p-, n-, p-, and n-type regions from the interface to the depth, and reference numerals 54, 55, and 56 denote signal charges which are generated in response to incident light and are accumulated in p-type semiconductor regions. The signal charges 54, 55, and 56 are transferred and read out by CCDs formed in a direction perpendicular to the plane of FIG. 13.
U.S. Pat. No. 5,965,875 “Color separation in an active pixel cell imaging array using a tripe-well structure” has also proposed a photo-detection device having a triple-stage photodiode structure. According to this proposal, terminals of respective photodiodes are formed in the semiconductor interface, and are connected to the gates of MOS transistors which serve as amplifiers, so that signals of the photodiodes are amplified and read out. FIG. 14 shows this invention. Referring to FIG. 14, reference numeral 57 denotes a p-type semiconductor substrate; 58, an n-type semiconductor layer formed to be stacked on the substrate 57; 59, a p-type semiconductor layer formed to be stacked on the layer 58; and 60, an n-type semiconductor layer formed in the semiconductor interface to be stacked on the layer 59. By pairing the substrate 57 and layer 58, the layers 58 and 59, and the layers 59 and 60, three photodiodes are formed. Reference numerals 61, 62, and 63 denote MOS transistors, which are respectively connected to the semiconductor layers 57, 58, and 59, and are used to amplify and read out signal charges accumulated on those semiconductor layers.
The above two proposals exploit the dependence of the light absorption coefficients of semiconductors on the light wavelengths to implement color separation. A photoelectric conversion unit for a shorter wavelength, i.e., blue light, is formed in the top layer, a photoelectric conversion unit for a longer wavelength, i.e., red light, is formed in the bottom layer, and a photoelectric conversion unit for a middle wavelength, i.e., green light, is formed in the middle layer, thus attaining color separation. These structures uses incident light without a loss, and output greater signals compared to a general color filter device. Moreover, since three different color signals can be extracted from a single location from two-dimensional viewpoint, the color resolution can be improved compared to the color filter system that extracts one type of color signal from one pixel.
However, the example of U.S. Pat. No. 4,613,895 uses embedded triple-stage CCDs which are stacked in the depth of a semiconductor, as described above. The structure therefore hardly control the potential wells of the middle and bottom CCDs by a gate electrode located on the surface so as to transfer signal charges.
In U.S. Pat. No. 5,965,875, since the electrodes of the respective photodiodes are commonly used as those of photodiodes neighboring in the depth direction, a signal voltage of one photodiode is influenced by that of another neighboring photodiode, and it is difficult to extract a independent signal of each photodiode. Furthermore, this prior invention offers another problem that considerable kTC noise resulting from the capacitance of the photodiode generates, upon resetting each photodiode